1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an internal combustion engine.
2. Description of the Related Art
An internal combustion engine is known, which comprises an electric purge control:valve for controlling the supply of purge gas fed into the intake passage of an engine from a charcoal canister, and an electric air bleed control valve for controlling the amount of air fed into the fuel passage of a carburetor. An electric current fed into the air bleed control valve is controlled on the basis of the output signal of an oxygen concentration detecting sensor (hereinafter referred to as an O.sub.2 sensor) arranged in the exhaust passage of the engine so that the amount of air fed into the fuel passage of the carburetor is increased as the amount of electric current fed into the air bleed control valve is increased (Japanese Unexamined Patent Publication No. 61-1857). In this engine, when the purge control valve is opened, and thus the supply of the purge gas is started, if the purge gas contains a large fuel component, an air-fuel mixture fed into the engine cylinders becomes extremely rich. As a result, the amount of electric current fed into the air bleed control valve is gradually increased so that an air-fuel ratio approaches the stoichiometric air-fuel ratio, and accordingly, the amount of air fed into the fuel passage of the carburetor is gradually increased. Subsequently, when the electric current fed into the air bleed control valve is increased to the maximum level of the controllable range, an air-fuel ratio control is changed from the air-fuel ratio control based on the air bleed control to the air-fuel ratio control based on the purge control, and thus the amount of purge gas is controlled so that an air-fuel ratio approaches the stoichiometric air-fuel ratio.
However, actually, when the supply of purge gas is started, the electric current fed into the air bleed control valve normally does not reach the maximum level of the controllable range, and thus, at this time, the amount of air fed into the fuel passage of the carburetor from the air bleed passage is gradually increased until the air-fuel ratio of air-fuel mixture fed into the engine cylinders becomes equal to the stoichiometric air-fuel ratio. However, if the amount of air fed from the air bleed passage is gradually increased as mentioned above, it takes a long time to equalize the air-fuel ratio with the stoichiometric air-fuel ratio. Consequently, since an extremely rich air-fuel mixture is still fed into the engine cylinders for a long time, a problem occurs in that a large amount of unburned HC and CO is discharged from the engine cylinders during that time.